Genalex KT88 Plate Dissipation Question

[Loren42]

I just received a new set of Genalex Gold Lion KT88s.

I am looking for the recommended plate dissipation for these tubes. Current B+ is about 505 to 510 VDC.

The cathode has a 10.16Ω*resistor and I measure the voltage across that resistor. That voltage is now set to 0.567 VDC.

I calculate the plate current as follows:

P (watts) = (505 VDC * .567 VDC) / 10.16Ω = 28 Watts

So I think that seems like a good target, but wanted to hear form others using this tube where they recommend to set them.

 

[vinylkid58]

Your amps output configuration will play a part in the idle current setting, so if you tell us, I'm sure you'll get a recommendation.

Appears to be fixed bias, class AB1 maybe? Tetrode or Ultra Linear?

jeff

 

[DF96]

What does the amplifier designer recommend? Too high and too low current can both cause distortion, although too high also shortens valve life.

 

[Loren42]

The amp is a home brew Williamson design using Edcore output transformers in ultralinear configuration.

Bias is fixed bias. I'll see if I can post a schematic.

I was using Ruby tubes, but one of them malfunctioned and its current dropped to zero. So, I had an excuse to order some Gold Lions and they arrived this afternoon and they are now burning in with the plate dissipation set to 28 Watts. ;-)

 

[Loren42]

Here is the schematic:

An externally hosted image should be here but it was not working when we last tested it.

 

[trobbins]

Your operating plate voltage would be a little down from 505V due to OT primary resistive drop. Your operating anode current would be a little down on cathode current due to idle screen current. They may reduce your actual plate dissipation by about 5-10% from what you initially calculated.

 

[Loren42]

Your operating plate voltage would be a little down from 505V due to OT primary resistive drop. Your operating anode current would be a little down on cathode current due to idle screen current. They may reduce your actual plate dissipation by about 5-10% from what you initially calculated.

Thanks! Yeah, that makes sense. I just wanted to get a feel if others are in agreement with my target dissipation.

I should measure the actual voltage on the plates when I get a chance, but I think it is very close to the 505 VDC figure.

Genalex rates these at 35 Watts maximum. I don't know what the new version of these tubes really are rated for, but 28 should be a safe bet, I would think.

 

[DF96]

With fixed bias I think it is generally best to run valves cooler than when using cathode resistor bias. Two reasons for this:
1. there is no automatic protection against thermal runaway, so best to be cautious
2. there is no bias shift with signal so you don't have to allow for this by running hot when quiescent